In the field of communications, data transmission techniques are known where an amount of data to be transmitted is divided into a plurality of data segments, the data segments being ordered in a sequence. These data segments are then transmitted in the order of said sequence.
This procedure occurs at a so-called sending peer of a communication governed by a predetermined protocol that contains the rules for handling such data segments. The receiving entity associated with the predetermined protocol is referred to as the receiving peer. The concepts of protocols, protocol hierarchies, layering, and protocol peers is well known in the art, see for example “TCP/IP Illustrated Volume 1, The Protocols” by W. Richard Stevens, Addison Wesley 1994. The well known Transmission Control Protocol (TCP) from the TCP/IP protocols suite is an example of such a protocol that arranges data to be sent into a sequence of segments.
Typically, in order to be sent, the individual segments will be placed into data units having a structure defined by the given protocol. These data units may have different names in the context of different protocols, such as packets, frames, protocol data units, cells, etc. In the present description the term “data unit” shall be used generically to cover any such defined data structure. The present specification shall use the terms “segment” and “data unit” interchangeably.
The sending peer will hand the data unit downwards to a lower layer, e.g. a TCP sending peer will hand a TCP frame down to a link layer, and on the receiving side the receiving peer shall receive data units from the lower layers. The predetermined structure defining the data units, e.g. defining a beginning and an end, allows the receiving peer to identify individual segments.
It may be noted that in accordance with the OSI layering concept, it does not matter how the data units passed to a lower layer are processed and transported there. Namely, the given sending peer passes a stream of bits downward and the receiving peer receives a stream of bits, where this stream of bits contains certain identifying elements, such as frame boundary indicators, with the help of which the receiving peer can identify individual data units and individual segments.
In order to ensure the reliable transmission of data, many protocols provide the feature of data unit retransmission, which means that segments from the sequence can be retransmitted if necessary. Typically this will be done with the help of an acknowledgement technique, which means that the correct receipt of a data unit by the receiving peer is acknowledged with an appropriate acknowledgement message that the receiving peer sends back to the sending peer. Once the sending peer has received such an acknowledgment message, it can appropriately continue sending further data units, or if no acknowledgement or a non-acknowledgement message is received, the data unit that was not correctly received by the receiving peer can be retransmitted by the sending peer.
Several mechanisms are known with the help of which a sending peer is supposed to obtain an indication that the loss of a data unit or segment has occurred, such that an appropriate retransmission can take place. One such known feature is retransmission time-out, which means that after sending a data unit, a timer is monitored, and if a predetermined amount of time passes without having received an acknowledgement for the given data unit, then it is assumed that the data unit has been lost and it is accordingly retransmitted.
Another such mechanism is that of counting duplicate acknowledgements. A duplicate acknowledgement is an acknowledgement that identifies as the last correctly received data segment a data segment that has already been acknowledged previously. Namely, many protocols, such as e.g. TCP, have an acknowledgement generating mechanism for receiving peers that operates to send out an acknowledgement message for each correctly received segment of the sequence, where the acknowledgement identifies the last correctly received data segment in the order of sequence. In other words, if for example the first to fourth data segments have been received and acknowledged, and then the fifth data segment arrives, the receiving peer will send out an acknowledgement for that fifth segment. If thereafter the seventh and eight segments correctly arrive, then the receiving peer will again send out one or two acknowledgement messages, but these acknowledgement messages will only identify the fifth segment, because the fifth was the last segment that was correctly received in the order of the sequence. Namely, the receiving peer is expecting the sixth segment, and even if it correctly receives segments above the sixth segment, it will continue to acknowledge the fifth segment. Consequently, the receipt of duplicate acknowledgements by the sending peer gives the sending peer an indication that a data unit has been lost.
As a consequence, in protocols that use the above described acknowledgement mechanism according to which acknowledgement messages only acknowledge the last data unit that was correctly received in the order of the sequence, even if data units are received that lie further on in said sequence, a retransmission mechanism may be implemented that performs a retransmission if a predetermined number of duplicate acknowledgements is received by the sending peer. In TCP, the corresponding mechanism is known as “fast retransmit”, where a retransmission is enacted after the receipt of three duplicate acknowledgements. A detailed description of the fast retransmit mechanism in TCP can e.g. be found in the above mentioned book by Stevens, chapter 21.7.